Automatic volume control



April 8,1941. D. E. FOSTER 2,237,421

AUTOMATIC VOLUME CONTROL Filed Sept. 3. 1938 p7 AMPLIFIER r0 (/T/L/ZA r/o/v 1 7 8 DEV/CE 2 3 4 2 ,1 w SIG/VAL 9 SOURCE 7'- :5

J DEMODULATOR {I 1 +5 .4 VC\ A V C L RECTIFIER nv ur RES/STANCE INVEN TOR. DUDLEY E. F05 TER ATTORNEY.

Patented Apr. d, 1941 AUTOMATIC VOLUME CONTROL Dudley E. Foster, South Orange, N. J., assignor to Radio Corporationof America, a corporation of Delaware Application September 3, 1938, Serial No. 228,290

6 Claims.

My present invention relates to automatic volume control circuits for radio receivers, and more particularly to an automatic volume control circuit utilized in conjunction with a controlled amplifier of the negative feedback type.

One of the main objects of my present invention is to improve automatic volume control (AVC) action in connection with high frequency amplifiers; the improvement essentially comprising the utilization of the input resistance of the amplifier for regulation of the amplifier tuned impedance thereby to aid the normal AVC action which is secured by variation of the amplifier transconductance.

Another important object of my invention may be stated. to reside in the provision of a signal transmission tube, and particularly one operating to transmit high radio frequencies, which is provided with sufficient negative feedback to impart a pronounced input resistance-bias characteristic to the transmission tube whereby the tuned impedance of the transmission tube may be widely varied in magnitude as the transconductance of the tube is controlled in accordance with signal amplitude variation.

Another object of the invention may be stated to reside in the provision of a signal amplifier, adapted to operate in a signal carrier frequency range above mc., and the amplifier being provided with negative feedback of substantial magnitude whereby variation of the transconductance of the amplifier over a wide range of values results in a like variation of substantial magnitude of the amplifier input resistance.

Still other objects of my invention are to improve generally the operation and efficiency of AVG circuits for amplifiers functioning to amplify currents of a frequency value of the order of 5 mc., and more especially to provide improved AVG circuits for receivers operated in the aforesaid range which are not only reliable in operation, but are economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing:

Fig. 1 shows a receiving system embodying the invention,

Fig. 2 graphically illustrates the functioning of the invention,

Fig. 3 shows a modification.

Referring now to the accompanying drawing, wherein like reference characters in the different figures indicate similar circuit elements, the signal receiving system shown in Fig. 1 is of a type commonly employed to receive modulated high frequencies in the 5 to me. range. Such a receiver may be, for example, of the superheterodyne type. In such a case the numeral I denotes the signal source, and is to be understood as comprising any desired type of signal collector device which feeds one or more tunable radio frequency amplifiers; the amplified signal being converted to an intermediate frequency by any desired type of converter network. The intermediate frequency energy may be chosen from a high radio frequency range, and preferably will be of the order 13 me. The I. F. energy is sub sequently demodulated by a second detector.

The numeral 2 will be understood as designating the usual transformer coupling the signal source to the tuned input circuit of the radio frequency amplifier. The tuned input circuit 3 of the electron discharge tube 4 feeds signals to the input electrodes of tube 4. Tube 4 will function as an amplifier of the selected modulated signal carrier energy. The signal carrier range may be of the order of 5 to 100 mc., and this will include channels employed for television transmission. The tube 4 may be of the 1851 type for example; such a tube is a pentode with a high mutual conductance, and is of particular value in amplifying signals in the television range. The direct current source B (not shown) establishes the screen and plate potentials at proper positive values. 'Resistor R is connected from cathode to ground; condensers 5 and 6 being direct current blocking condensers. The signal grid is connected to the high alternating potential side of tunable input circuit 3. The low potential side is connected to the AVG rectifier. It will be understood that numeral 3 denotes the usual variable tuning condenser employed to tune the signal circuit over the desired frequency range. The reference character R1 designates the grid to cathode, or input, resistance of tube 4; the resistance being represented in dotted line. This input resistance is in shunt across the tuned impedance of circuit 3; hence it affects the loading of the latter, The amplifier 4 may be followed by a tunable circuit 1 which is similar to circuit 3. Network 8, which is fed by circuit 7, may comprise further radio frequency amplifiers; the amplified signal will then be converted to the I. F. energy and amplified at I. F. Network 8 may be the first detector and I. F. amplifier circuits. The demodulator, or second detector, 9 is fed with the I. F. energy, and the utilization device may comprise one or more ampiifiers arranged to amplify the modulation frequencies, whether audio or video, prior to reproduction. Those skilled in the art are fu ly aware of the manner of constructing receiving systems of the general type described.

The AVC rectifier, which may be a diode coupled to the input of the second detector, produces the direct current voltage, or AVC bias, which is used to control the gain of the amplifier tube '1. The lead I 0, provided with a proper filter denoted by numeral H, impresses the AVG bias on the signal grid of amplifier 4. The carrier amplitude at the input of the second detector is maintained substantially uniform regardless of a wide range of variation of the signal carrier amplitude at the signal collector. Those skilled in the art are fully aware of the functioning of the AVG circuit, and realize that as the carrier amplitude at the AVG rectifier increases, the magnitude of the AVC bias increases thereby reducing the gain, or transconductance, of the controlled amplifiers. Of course, the AVG bias may be applied to as many of the signal transmission tubes as is desired.

It is highly desirable, when operating in the 5 to 100 me. range to aid the AVG action. In the present invention this is accomplished by developing carrier voltage across the cathode resistor R, and impressing this carrier voltage upon the signal grid of tube 4 in degenerative phase with the signal carrier voltage originally impressed thereon from the signal source. It will be observed that the resistor R is un-bypassed for carrier frequencies, and, hence, carrier voltage is developed across the resistor. develops some portion of the no-signal D. C. bias for the signal grid.

As is well known, there is a resistive component of the input impedance of an electron discharge tube which decreases as the square of the frequency, and which component is due to the transit time of the electron flow. This resistance becomes low enough to be an important factor in the tuned circuit impedance of amplifiers operating above 5 mo. plifier 4 is a function of the transconductance Sm of the tube, and is low at high Sm values whereas it is high for low Sm values. Hence, the resistance R1 is affected by the AVG action, since the latter varies the transconductance of tube 4. In Fig. 2 there is shown the relation, in a purely qualitative manner, between the input resistance R1 and the increase in negative bias of the tube; the cathode resistor R being assumed to be zero. It will be observed that the input resistance R1 varies in a sense such that it actually opposes the AVG action insofar as the loading effect of R1 on tuned circuit 3 is concerned.

By inserting the cathode resistor R, and by proper choice of the magnitude thereof, there is secured the reverse characteristic for input resistance variation, as is depicted in Fig. 2. It can be shown that the input resistance, regardless of its magnitude, of a vacuum tube with an unbypassed resistor in the cathode is composed of two Of course, the resistor R The input resistance R1 of amterms, a positive term independent of transconductance and a negative term varying in magnitude with transconductance. The transit time resistance of the tube varies with transconductance likewise. It is important here to distinguish between the input, or grid-cathode, resistance of the tube which is due to transit time and varies with Sm, and the resistance appearing across the input circuit. The latter is the important one from consideration of gain and selectivity due to an input tuned circuit and is the one which is affected by cathode impedance, although it is of course physically due to the transit time phenomenon in the tube.

Since there are two terms which vary with Sm, a positive term due to transit time resistance per se, and a negative term due to the cathode resistor, by proper choice of the cathode resistor, the two terms may be made substantially equal for all values of Sm. If the cathode resistor is increased above this value the direction of variation of input circuit resistance with Sm is reversed, the higher the cathode resistance the steeper the slope of change in resistance with bias (i. e. Sm). The magnitude of cathode resistance which will produce equality of positive and negative terms which vary with Sm is a function of tube parameters; in general, having a much greater magnitude for low Sm tubes than high Sm tubes. For example in an 1851 type tube it is about 40 ohms, whereas in a 6K7 tube it is about 300 ohms.

If the cathode resistor is made too large in an attempt to obtain a steep reverse slope of resistance vs. bias variation, oscillation will ensue. Consequently the value to secure a desirable AVG characteristic, should be larger than that value which causes the resistance appearing across the input tuned circuit to be in variable with bias, but smaller than the value which causes osciilation. Upon assigning a value of approximately 7 5 ohms to the resistance R in the case of an 1851 type tube, and maintaining the latter unby-passed, there is secured a decrease of the input resistance R1 with increase of negative bias upon the signal grid of amplifier 4. In this case the tuned impedance of circuit 3 is a maximum when the transconductance of tube 4 is a maximum, and, therefore, the AVG action is augmented. This will be readily seen when it is realized that for weak signal reception the AVG bias applied to tube 4 is a minimum, and hence the input resistance RI is a maximum. The equivalent series resistance in circuit 3, due to the shunt resistance R1, is a minimum, and hence there is substantially no loading of circuit 3. As the AVG bias is increased the loading effect, due to the decrease of the shunt resistance R1, begins to increase materially. It has been found readily possible to ob-- tain as much as an eight-fold improvement in AVC action at 50 me. by use of the present invention.

Of course, it is to be understood that the magnitude of the resistor R will depend upon the desired steepness of the characteristic and the particular type of amplifier tube used. It is to be clearly understood that the magnitude and constants enumerated are by no means limiting. In some cases it will be possible substantially to decrease the value of resistor R, and yet secure the desired type of characteristic. For example, in Fig. 3, there is shown an amplifier including tube 4 and a cathode resistor R. The resistor R may be made less than a predetermined value of cathode resistor which gives a decreasing input resistance characteristic with increase of AVG bias;

this is accomplishedby connecting between the plateand cathode of tube 4 a capacitative reactance 12, the reactance being eifectively in series with R. For example, if the characteristic shown in Fig. 2 for R equal to 75 ohms is desired, it is possible to replace the resistor R by a resistor R. having a substantially smaller magnitude, provided the plate to cathodecapacity I2 is utilized. The latter capacity may be approximately 5 to 25 mmf. By Way of illustration it is pointed out that a substantial decrease of input resistance of tube 4 is secured with increase of negative bias on the signal grid when the value of resistor R. is approximately 33 ohms and the capacity I2 is chosen to have a value of 25 mmf.

A condenser from plate to cathode when taken in conjunction with the inherent grid-cathode capacity of the tube may be considered as a regenerative circuit. The amount of regeneration obviously depends not only on the magnitude of the capacities, but also on the magnitude of the cathode resistor since that. partially shunts the plate-cathode condenser for radio frequency currents. The regeneration produces a negative component of input circuit resistance. If the regeneration is too high the circuit will, of course,

oscillate. However, a smaller cathode-plate capacity than the value required to cause oscillation will give a negative term to the input circuit resistance in addition to the negative term mentioned above due to the cathode resistance. Consequently, it may be seen that if the negative resistance term due to plate-cathode capacity is present the negative term due to cathode resistance may be smaller for the same efiect on input circuit resistance.

While I have indicated and described several systems for carrying my invention into efiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a signal receiving system of the type employing a tube provided with a tuned input circuit, means impressing high frequency signals upon said tuned input circuit, an unby-passed resistor in the space current path of said tube for developing signal voltage thereacross which is in degenerative phase with the signal voltage impressed on the tuned input circuit, means impressing the degenerative voltage upon the tube input electrodes thereby to provide directly in shunt with the tuned circuit a substantially high magnitude of input resistance effect due to said tube, means responsive to an increase in signal amplitude for simultaneously decreasing the gain of the tube and the magnitude of said shunt input resistance efiect, and said resistor having a magnitude such that said resistance effect is substantially a maximum at substantially maximum gain of said tube.

2. In a signal transmission system, a tube having input and output electrodes, a tuned input input resistance of said tube directly in shunt withsaid input circuit and thereby minimize the loading efiect oi the said resistance on said tuned input circuit, and means responsive to carrier amplitude increase for simultaneously decreasing the transconductance of said tube and the magnitude of said effective input resistance thereby to decrease the signal transfer efficiency of said system.

3. In combination with a tube provided with at least a cathode, signal grid and output electrode, a tunable input circuit connected between the grid and cathode, a utilization network coupled to the output electrode, a source of modulated signal carrier energyywhose carrier frequency is in the 5 to me. range, coupled to said input circuit, a resistor connected in the space current path of said tube and developing carrier voltage thereacross which is impressed between the grid and cathode in degenerative phase, and an automatic volume control circuit responsive to carrier amplitude variations in said utilization network for varying the gain of said tube and simultaneously adjusting the magnitude of the effective tube input resistance directly in shunt with the input circuit in an inverse sense with respect to the carrier amplitude increase.

4. In a high frequency signal transmission network of the type comprising a tube having at least a cathode, signal grid and output electrode, a tuned signal input circuit connected between the grid and cathode, an output circuit, and means for adjusting the gain of the tube over a substantial range of values; a device for augmenting the action of said gain adjusting means comprising a resistive impedance connected in the space current path of said tube and developing signal voltage thereacross in degenerative phase with the signal voltage at said input circuit, means for impressing said degenerative Voltage upon said signal grid thereby to cause the input resistance effect of the tube directly in shunt with said input circuit to assume a substantial value, and said impedance having a magnitude sufficiently large to cause said resistance effect to vary directly with said gain adjustment over said range.

5. In a high frequency signal transmission network of the type comprising a tube having at least a cathode, signal grid and output electrode, a tuned signal input circuit connected between the grid and cathode, an output circuit, and means for adjusting the gain of the tube over a substantial range of values; a device for augmenting the action of said gain adjusting means comprising a resistive impedance connected in the space current path of said tube and developing signal voltage thereacross in degenerative phase with the signal voltage at said input circuit, means for impressing said degenerative voltage upon said signal grid thereby to cause the input resistance efiect of the tube directly in shunt with said input circuit to assume a substantial value, said impedance having a magnitude sufficiently large to cause said resistance effect to vary directly with said gain adjustment over said range, and said gain adjusting means consisting of an automatic volume control arrangement for decreasing the gain of said tube in response to increase of the signal amplitude level over a desired value.

6. In a high frequency signal transmission network of the type comprising a tube having at least a cathode, signal grid and. output electrode, a tuned signal input circuit connected between the grid and cathode, an output circuit, and means for adjusting the gain of the tube over a substantial range of values; a device for augmenting the action of said gain adjusting means comprising a resistive impedance connected in the space current path of said tube and developing signal voltage thereacross in degenerative phase with the signal voltage at said input circuit, means for impressing said degenerative voltage upon said signal grid thereby to cause the input resistance effect of the tube directly in shunt with said input circuit to assume a substantial value, and said impedance having a magnitude sufiiciently large to cause said resistance efiect to vary directly with said gain adjustment over said range, said impedance consisting of an unby-passed resistor, and a condenser connected between the cathode and output electrode for reducing the required magnitude of the resistor.

DUDLEY E. FOSTER. 

